The present invention relates generally to processes for manufacturing sintered varistors which are composed primarily of zinc oxide, and pertains particularly to heat treating of such varistors after they are sintered.
Zinc oxide varistors are highly non-linear and are especially suitable for overvoltage protection devices, such as overvoltage surge or lightning arresters. They contain mostly zinc oxide with certain selected additives for controlling the mechanical and electrical characteristics of the varistor. The varistors are generally in the form of rods or discs which are provided with metal electrode layers on the end faces.
The manufacturing process for zinc oxide varistors includes pressing a powder mixture of zinc oxide and the desired additives in a die to form a self-supporting body. Then the body is sintered in a furnace at about 1200.degree. C. (Celsius) for a time until a non-porous ceramic is formed, and is then cooled and provided with metal electrodes.
While as yet the mode of operation of zinc oxide varistors is not fully understood, a number of parameters in the manufacturing process are known to have a significant, and in some cases critical, effect on the electrical characteristics of the finished varistor. Two electrical characteristics of special importance for arrester varistors are exponent and stability. The term "exponent" as used herein refers to the value of the current-voltage characteristic exponent n of the voltage V in the current-voltage relationship I=KV.sup.n for a resistor, where I represents the current through the resistor and K represents a constant. The term "stability" as used herein refers to extent to which the constant K remains constant when the varistor is subjected for an extended time to an applied voltage low enough to prevent heat damage to the varistor by the leakage current.
Efforts to control the exponent of zinc oxide varistors by selecting the appropriate additives and sintering conditions have met with considerable success. Efforts to develop a stable varistor for use at high voltages have met with more limited success. It is known that the leakage current through a given zinc oxide varistor can be made more stable by subjecting it to an additional reheating cycle after it has been sintered. The reheating cycle involves a reheating to about 700.degree. C. for about two hours and removal from the furnace for rapid cooling in room ambient.
While the above reheating cycle does result in varistors with a more stable leakage current, it unfortunately leaves them with a leakage current of much greater magnitude for a given voltage than was exhibited by them prior to the reheating. Thus, in a sense it appears to simply accelerate the time-dependent increase of leakage current which characterizes long term varistor instability. Such increased leakage current makes the varistors unacceptable for high voltage surge arrester use without the additional provision of series gaps, because it will result in excessive heating of the varistors at the normal operating voltages with subsequent degradation of their other characteristics.